The present invention relates to an image recorder of the kind using an electrophotographic procedure and, more particularly, to an image recorder capable of producing a clear-cut image having a predetermined line width at all times with no regard to its operating conditions and ambient conditions by causing the line width of an image into coincidence with a predetermined reference width.
A prerequisite with a laser printer, facsimile machine, electrophotographic copier or similar electrophotographic image recorder is the stable image reproducibility. A current trend in the image recorders art is, therefore, toward a digital and high-density architecture. In parallel with this trend, there is an increasing demand for the high reproducibility and stability of a line width which is the basis of character images and graphic images, i.e., the reproducibility of a minimum pixel density. It has been reported that a line image or a dot image available with an electrophotographic image recorder undergoes a change in line width due to changes in various factors such as operating conditions and ambient conditions. Typical of such factors are the sensitivity of a photoconductive element, or image carrier, which varies with temperature, and the surface condition of the same which is susceptible to ozone and other harmful gases generated by various chargers. Another factor having influence on the line width is the particle size of a toner which is used as a developer. The change in line width is apt to thicken, deface, thin or blur reproduced characters and lines.
A printer with an implementation for adjusting the line width and density of an image has been proposed, as disclosed in Japanese Patent Laid-Open Publication (Kokai) No. 61-25161. The printer taught in this Laid-Open Patent Publication has an exclusive switch provided on an operation board and accessible for selecting a line width. The amount of laser light is controlled on the basis of the selected line width. Specifically, based on the fact that the line width decreases with an increase in the amount of laser light and increases with a decrease in the laser light, the printer controls the line width of an image. The operation board is also provided with another exclusive switch assigned to density. This switch is operable to change the voltage to be applied to a charger and thereby the image density, the voltage increasing with the selected density.
Japanese Laid-Open Patent Publication No. 63-67073 discloses a laser light modulation system which enhances reproducibility by preventing reproduced images from being defaced or blurred due to thickening or thinning of the line width. Generally, black thin lines and white thin lines tend to thicken themselves in an image scanning type system and a background type system, respectively. In the light of this, the modulation system shown and described in Laid-Open Patent Publication No. 63-67073 reduces the amount of laser light when a pixel of interest corresponds to such a thin line, thereby achieving a desired thin line. Further, white thin lines and black thin lines tend to thin themselves in the image scanning type system and the background type system, respectively. To cope with this tendency, the modulation system reduces the amount of laser light which indirectly forms a pixel of interest.
While the printer taught in Laid-Open Patent Publication No. 61-25161 allows one to select a desired line width and a desired density by manipulation, it cannot control the line width and density to predetermined reference values automatically in association with the operating conditions and ambient conditions. On the other hand, the system disclosed in Laid-Open Patent Publication No. 63-67073 reduces the amount of laser for a thin line which is produced by the irradiation of laser light and apt to thicken, while increases the amount of laser light associated with pixels around the thin line for a thin line which is produced by the non-irradiation of laser light. This system, therefore, does not contemplate to eliminate changes in light width ascribable to the changes in the characteristics of a photoconductive element and developing device which are caused by the varying operating conditions and ambient conditions, i.e., the idea is not to maintain the line width after development constant. With the proposed system, it is impracticable to control the line width to a reference value automatically in association with the operating conditions and ambient conditions.